The cause of Epizootic Hemorrhagic Disease (EHD) is an Orbivirus that is carried by deer. There are 2 serotypes of the virus in deer. There are also a number of related viruses within the Bluetongue Virus group that can cause similar disease in livestock. Additionally, there are several viruses in the Adenovirus group that can cause similar disease signs in deer.

How is the Virus Spread?

EHD virus resides in a small portion of the deer population. It is spread from deer to deer by Culicoides spp. gnats. There are three major driving forces in EHD outbreaks – 1) deer population and density, 2) level of immunity of deer to EHD, and 3) gnat population. In Idaho, about 10-15% of the deer sampled around the state have antibodies to EHD and thus are likely immune. However, the converse, about 80% of the deer in the state have no immunity to EHD. Gnat populations expand greatly during times of warm to hot weather that result in lots of wet areas due to receding water lines in ponds, streams, wet meadows, pastures etc. Deer also tend to concentrate around water and shaded areas in hot weather. Since lots of deer are not immune to EHD and the virus is in the population, and given high deer populations and deer densities, virus transmission via gnats is relatively efficient and results in spread of the virus to the majority of deer in a given area. If the level of herd immunity is low, many deer can get sick in a short time period which may lead to a disease outbreak with high morbidity and mortality.

Can the Gnats be Controlled to Prevent the Disease?

The gnats that can spread EHD are common and widespread in the landscape. Appropriate breeding habitat is moist soil to shallow stagnant water including ponds, creeks, seeps, springs, flooded pastures, water filled hoofprints, puddles, sprinkler overflow, water trough overflow, etc. The gnats can also fly up to a mile from their breeding sites. Spraying for gnat control is difficult since the breeding areas are common and widely dispersed. In addition, a broadcast insecticide could potentially harm beneficial insects that are so important for many species of wildlife and fish.

Can Humans, Domestic Livestock or Other Wildlife be Infected With EHD?

Members of the Cervidae family (deer, elk, moose, reindeer) are the most susceptible. Mule deer and white-tailed deer are the primary known hosts of this virus. Elk have been documented with EHD occasionally. Reports of disease in moose and caribou are largely non-existent other than in zoo settings. Large scale outbreaks in pronghorn are documented in Wyoming.

Livestock are likely exposed to EHD during outbreaks and within certain gnat populations. However, only cattle have been documented to be clinically affected by EHD and then only very rarely with a very mild clinical course that lasts a few days with lethargy, low grade fever and some loss of appetite.

EHD does not affect humans.

How Common is the Disease?

EHD is very common in deer in the USA, especially in the southeastern and the lower central parts of the US. The Midwest and northeastern parts of the US are affected sporadically with relatively large scale outbreaks of disease. In the west, outbreaks are known in most states, but the size and scale varies widely.

In Idaho, EHD has occurred before and will likely occur in the future. Small outbreaks are known to have occurred in the Weiser and Peck areas and a large outbreak occurred in 2003 along the Clearwater River between Orofino and Kamiah.

Can Infected Deer be Treated?

No effective treatment exists for viral infections in animals or humans. The logistic demands for effective treatment of a wild population make the feasibility of a treatment program very difficult. If an effective treatment were available, the treatment would have to applied to the majority of deer in the area for an appropriate treatment interval. With wild deer, there is no way to ensure that the majority of deer get treated in a timely manner. In addition, use of drugs in animals requires establishing an appropriate meat withdrawal time period prior to human consumption which makes treatment near or during the hunting season problematic.

How Long Does an EHD Outbreak Last?

EHD generally takes one of several potential scenarios in deer. Acute death can occur in 1-2 days following infection due to overwhelming viral infection and massive physiological changes including pulmonary edema or fluid in the lungs.

Chronic infections can occur if lower levels of virus are present and may result in lung damage that leads to bacterial colonization and subsequent pneumonia in the weeks to months following infection. In addition, because the virus attacks cells lining the blood vessels, erosions in the mouth and the gut wall may cause an animal to stop eating for days to weeks which may lead to nutritional deficiency.

Another long term sequel that can occur in chronically infected deer is deformity of the hooves. The virus attacks the coronary bank, hoof growth is disrupted, the animal becomes lame and may over time, develop abnormalities in hoof growth. Hoof abnormalities may show up a year after infection in deer that recover from the initial infection.

Since virus transmission is dependent on gnats, anything that decreases the gnat population will slow the rate of transmission. Gnats are not cold tolerant and frost will end the gnat life cycle. Outbreaks often continue until the first killing frost of the fall.

Areas of low deer density or areas devoid of deer between small populations of deer will also slow or stop the infection due to inefficient virus transmission.

How Many Deer Have Died From this Disease?

Estimates of deer mortality are limited. IDFG personnel estimate that approximately 150-200 carcasses are known or have been removed from the area. It is reasonable to estimate that carcass recovery is less than actual mortality. The actual death losses will never be known with certainty, but are likely in the hundreds. Deer counts this winter will provide a much better picture of the remaining deer population.

Can EHD Spread to Other Deer Populations?

Possibly. High deer numbers and density in an area leads to very efficient transmission of the disease. Given the flight capabilities of gnats, the disease can spread miles at a time.

Can the Presence of EHD Affect the Hunting Season?

Locally, deer populations that experience an outbreak of EHD tend to be reduced, possibly severely. Outside of known affected area, deer populations should be unaffected and hunting should be similar to previous years.

Hunters are advised that they should not harvest obviously sick deer. The virus cannot infect humans so even in areas where EHD is present, consumption of meat from animals that are obviously not sick poses no significant risk to human health.